A Guide to Patch-Burn Grazing for Biological Diversity

Patch-burn grazing is getting a lot of attention from a wide variety of audiences these days.  The management system has generated substantial enthusiasm among some people – particularly those interested in improving habitat for prairie wildlife species.  It has also generated substantial skepticism among others – particularly those concerned about potential negative impacts of grazing in eastern tallgrass prairies.

To date, the majority of research on and demonstration of patch-burn grazing has been oriented toward influencing agricultural grassland management because of the system’s potential to improve wildlife habitat at those sites.  Research has largely shown that farmers and ranchers can reap the habitat benefits of patch-burn grazing without compromising current stocking rates or livestock performance.  Because wider adoption of patch-burn grazing techniques on private lands could have tremendous positive impacts on prairie wildlife populations, those research results are exciting, and the continuation of work toward that objective is very appropriate and important.

However, the emphasis on developing and testing patch-burn grazing as an agricultural tool has also somewhat limited our ability to evaluate the system’s impacts (current and potential) on biological diversity.  Variables such as stocking rates, timing and intensity of grazing, fire season and frequency, and others tend to be defined such that they are compatible with local grazing systems and cultures.  Again, this is very appropriate, but it also narrows the range of possible modifications to the basic patch-burn grazing model that are being tested.

In contrast to research projects oriented toward balancing agricultural production with wildlife habitat, our work with patch-burn grazing in the Platte River Prairies of Nebraska is targeted only at building and sustaining biological diversity.  Our stocking rates, fire frequency, and other variables are all primarily designed to favor plant diversity.  Because few others are experimenting with patch-burn grazing from the same standpoint, we have tried to gather as much data as we can about the impacts of our various patch-burn grazing modifications – especially related to impacts on plant species and plant communities.  We’re taking the approach that if we can learn how to optimize management for plant diversity and overall biological diversity, we can then help translate and modify those methods to fit the specific needs of private landowners, public land managers, and others.

A burned patch in restored prairie along the Platte River in Nebraska. Cattle are concentrating their grazing in this area, but the light stocking rate employed means that while many grasses are being grazed, most forbs are not. In this case, ungrazed forbs include opportunistic plants such as black-eyed susan and hoary vervain, but also compass plant and Canada milkvetch, which are often viewed as negatively affected by cattle grazing.

This week I attempted to capture what we’ve learned about patch-burn grazing to this point and incorporate it into a single document.  We certainly don’t have all the answers, and the document should not be read as a prescription for precisely how to employ patch-burn grazing.  Rather, it is a description of the kinds of things we’re trying on our sites and the results we’ve seen so far.  I hope it will stimulate thought and discussion among other prairie managers looking for new ideas and options for dealing with prairie management challenges.

As I’ve said before on this blog, patch-burn grazing is not an appropriate management system for all prairies.  Moreover, patch-burn grazing often gets treated as a single narrow management system, but should really be seen as a basic template – and should rarely be implemented without tailoring it to meet particular objectives.   This new document describes some of the modifications of that template being tested at our sites and the responses we’ve seen from the prairie communities we manage.

If you’re interested, you can download the document here.

Why Grassland Birds are Poor Indicators of Prairie Quality

I presented this argument to a Nebraska symposium on grassland birds in 2008 and managed to escape relatively unscathed.  Now I’m testing my luck with a wider audience.  At least no one can throw things at me through the computer…

Let me start by saying that I’m a big fan of birds.  I really enjoyed working on my graduate research, which focused on grassland birds and their vulnerability to prairie fragmentation.  I also think birds are generally pretty and interesting. However, the truth is that prairie birds make up only a tiny percentage of the species in prairies (most of which are invertebrates, followed by plants).

Grassland birds make up a tiny percentage of the species living in a prairie - the vast majority of which are invertebrates and plants.

However, grassland birds are often held up as indicators of whether or not a prairie – or a prairie landscape – is “healthy” or “high quality.”  A common refrain in prairie conservation goes something like this; “If we have our full complement of grassland birds in this prairie and/or landscape, it’s a good bet that all the other species are also doing well.”  Unfortunately, while prairie birds are relatively easy to study and monitor, they may not do a good job of reflecting how the rest of the prairie is doing.  Let’s look at some of the most important attributes of prairies and some of their major threats – and consider how well birds correlate with them.

Species Needs – Survival, Reproduction, and Dispersal.

First and foremost, species have to survive and reproduce in order to persist in a prairie.  This applies to every species, from large vertebrates to tiny invertebrates and the entire suite of plants.  It’s important for us to know that grassland birds are surviving and reproducing, but can they tell us whether other species are doing the same?  You could argue that because they eat insects, grassland birds could have an impact on the survival of some insect species.  That’s true to a point, but grassland birds are generalist feeders – they tend to eat whatever insects are easiest to catch at any particular time – so while the abundance of grassland birds might impact the overall abundance of insects, you can’t really tie the presence of a particular grassland bird species to the survival of a particular insect species (or vice versa).  In other words, the plight of a rare leaf hopper or butterfly species is unlikely to be correlated with grassland birds.  Nor are grassland birds good predictors of plant species survival – the presence of meadowlarks or Henslow’s sparrows tell us nothing about whether or not compass plant or leadplant is thriving.  Grassland birds require certain habitat structure types (short vegetation, tall/dense vegetation, etc.) but they don’t much care whether that vegetation consists of smooth brome and sweet clover or a large diversity of native plants.

Henslow's sparrows are a bird of conservation concern and their presence in a prairie can be seen as a conservation success. However, although they tend to require fairly large prairies, and can indicate the presence of certain vegetation structure types, they don't indicate whether or not a prairie has a diverse plant or insect community.

In addition to basic survival, animal and plant species need to be able to move around the landscape in order to recolonize places where they have disappeared, and to maintain genetic interaction between populations (important for genetic diversity).  In landscapes where prairies exist as isolated remnants, moving between prairies becomes very difficult.  Corridors of prairie vegetation between prairies become important in those landscapes, and prairies near each other provide better opportunities for interaction within species than do more isolated prairies.  Because most grassland birds fly south at the end of each season and return the next year, (and the ones that don’t can still fly long distances between prairies) they don’t rely on those physical connections between prairies like most other animals and plants do.  Since grassland birds are pretty unique in terms of long-distance flying ability, they are a poor indicator of conditions that affect less mobile species.

Ecological Services and Ecological Function – Pollination, Seed Dispersal, etc.

Apart from the needs of individual species, prairies rely on certain processes to keep everything humming along.  Pollination and seed dispersal are two good examples.  Both affect the viability of prairie plant species, and neither has much to do with grassland birds.  Pollination primarily relies on plant diversity and bees – the most important pollinator group in prairies – and both plant diversity and bees are pretty disconnected from grassland birds.  We don’t know much about the role of prairie birds as seed dispersers, but it’s a good bet that they do very little seed dispersal during the summer when they’re primarily eating insects.  The role of migrating grassland birds as seed dispersers would be an interesting thing to study – but our use of grassland birds as indicators of prairie quality is always based on their presence during the breeding season.  If you were going to measure whether or not pollination and seed dispersal were functioning adequately, you’d likely evaluate the diversity of plants, the abundance of bees, and some of the potential obstacles to seed dispersal (tree lines, isolation of prairies, etc.), but I don’t think measuring grassland birds would tell you much.

Resilience – Redundancy and the Ability to Withstand Stresses and Invasive Species.

One way to think about the resilience of a prairie is as a measure of how well the prairie can bounce back from stresses.  For example, species diversity adds resilience to a prairie because when many species are present – especially when they overlap in the roles they play – the loss of an individual species can be a relatively minor blow.  If there are dozens of bee species pollinating flowers in a prairie, a disease that wipes out one or two species will probably not have a huge impact on seed production.  A diversity of plant species can also help to dampen the impacts of an event such as a severe drought or intensive grazing that temporarily weakens the vigor and growth of dominant plant species.  When there are lots of plant species present, the weakening of some leads to increased growth and abundance of others.  This helps maintain a stable supply of food for herbivores, and also helps prevent encroachment by invasive species that might otherwise take advantage of the weakened plant community.

Grassland birds may help bolster the resilience of a prairie in some ways.  They might, for example, help suppress an outbreak of grasshoppers by focusing their feeding on that easy-to-find prey species, and thus limit its abundance.  However, as discussed earlier, they don’t have much to do with pollination, nor would they help provide food for herbivores during a drought.

If you want pollination you need bees, not birds.

Threats to Prairies – Habitat Fragmentation, Invasive Species, Broadcast Herbicide Use, and Chronic Overgrazing.

One of the best arguments for grassland birds as an indicator of prairie health is that they are vulnerable to the loss and fragmentation of grassland habitat.  This is true.  A diverse and successfully reproducing community of grassland birds requires relatively large and unfragmented grassland.  In addition, because some grassland breeding birds need short vegetation and others need tall/dense vegetation, a diversity of birds can indicate a diversity of available habitat structure  – and that’s important to many other wildlife species as well.  Greater Prairie Chickens are often promoted as particularly good indicators because they are a single species that needs both large grasslands and a diversity of habitat structure.

However, there are a couple of other things to consider.  First, while some grassland bird species need large prairies, we don’t really know whether the minimum area required by grassland bird species is larger or smaller than that required by plant or insect species.  We know that many prairie plant species have survived for a very long time in tiny isolated prairies.  But because individual plants of many species can live almost indefinitely, due to their ability to generate new plants through rhizomes and other asexual means, the populations of plants in those tiny prairies could be in a death spiral due to the lack of genetic interaction with other populations.  You could make the argument that plants need much larger prairies than birds (thousands of acres, perhaps?) in order to maintain genetic fitness.  We simply don’t know.  And we know even less about the prairie size needs of insects.

Second, while grassland birds do require fairly large grasslands, most don’t actually require PRAIRIES.  Prairie chickens and many other grassland bird species have benefitted greatly from Conservation Reserve Program (CRP) fields that have added large numbers of acres of switchgrass, brome, and low-diversity grasslands to agricultural landscapes.  However, those same CRP fields have done very little for native wildflower populations or pollinator insects (or other insects that rely on diverse communities of native plants) so the increase in grassland birds in landscapes with CRP doesn’t really tell us much about the health of most other prairie species.

This smooth brome-dominated grassland is of very little to most prairie species, but would provide relatively good habitat for some grassland bird species (except for the trees in the background).

Because grassland birds can live comfortably in grasslands made up of a few native grasses, or even non-native grasses, they are poor indicators of the impacts of most invasive species – a major threat to prairies.  Similarly, broadcast herbicide use that greatly reduces the number of plant species in a prairie has little impact on the grassland birds nesting there.  Finally, a prairie that is being overgrazed would certainly have different bird species than one that is not being grazed at all, but a prairie that was chronically overgrazed for decades – and then managed well again – would have a pretty low number of prairie wildflower species but a very nice-looking grassland bird community.

One threat that grassland birds can be an excellent indicator for is tree encroachment on prairies.  Most grassland birds avoid nesting anywhere near even a solitary tree, let alone a grove of them, so a prairie with few birds could indicate a tree problem.  On the other hand, it’s pretty hard to miss a bunch of trees growing in a prairie…

Summary

Here’s the real point.  Grassland birds are an important component of prairies.  A prairie without all of its appropriate prairie bird species, or in which those species are not successfully raising broods, is missing something valuable.  Improving grassland bird success in prairie landscapes is an important and worthwhile objective.  At the same time, however, a prairie that has a full complement of successful grassland bird species doesn’t necessarily have diverse plant and insect communities, functioning ecological processes, or a low risk of invasive species or other threats.  In other words, grassland birds are an important component of high quality prairies, but their presence and/or success doesn’t necessarily mean a prairie is high quality.

Now that I’ve spent 1,500 words bashing prairie birds (I really do like birds…) the relevant question is, “What SHOULD we use as indicators of prairie conservation success?”  I wish I had a simple answer.  Part of the answer, of course, depends on how you visualize prairie quality (see my earlier post on this subject) because evaluation needs to reflect objectives.  But if our vision of a high-quality prairie includes species diversity, habitat heterogeneity, and other complexities, our evaluation methods will have to be complex as well.  Figuring out how to “take the pulse” of prairies may be the most important conservation challenge we face, because without that information we can’t design effective conservation strategies.

While we still have a lot to learn about how to take that pulse, it’s clear that we’ll have to do more than just count birds…